(i) Field of the Invention
This invention relates to the application of an electrochemically active paste to battery grids and, more particularly, relates to a method and apparatus for continuously mixing and producing an electrochemically active paste for continuous application to a battery grid in a discrete or continuous form for use in lead-acid battery systems.
(ii) Description of the Related Art
U.S. Pat. Nos. 6,886,439 and 7,007,579 granted May 3, 2005 and Mar. 7, 2006 respectively to Teck Cominco Metals Ltd. disclose a method and apparatus for continuously producing positive and negative electrode plates from continuous metal mesh freshly pasted with an electrochemically active paste without the use of paper barriers to produce battery plates for use in lead-acid batteries, both patents incorporated herein by reference. The battery paste typically is produced by a batch process in which ingredients comprised of dry particulate lead oxide, water and sulphuric acid, along with other ingredients, are mixed together to produce a paste of desired viscosity to saturate cavities in the mesh strip and to coat and adhere to the opposite sides of the mesh strip.
Batch production of battery paste, which typically requires 20 or more minutes of mixing, inherently results in lack of uniformity of viscosity and ingredient composition. Due to conventional batch mixing techniques, exothermic heating resulting from chemical reactions within the paste is difficult to control, producing localized high paste temperatures. Batch processes typically are operated at about 60° C., with application of paste at about 49° C.
It is common for lead oxide conveyed by various means in the manufacture of lead acid batteries to become clumped and crusty prior to entering into the paste mixing system. These random clumps can be friable or hard impinged material from caking on the pipes or apparatus or from wet and reacted material in a near clinker state because of lead oxide contact with water due to leaks in the system or due to presence of condensation. The clumped material can not easily be screened or separated from the product stream, since any removal device is quickly plugged by the inherent stickiness of the powder, causing increased maintenance problems and increased down time. Conventional batch mixing processes rarely breaks these clumps and, even when broken, the pieces are not sufficiently small to pass through the subsequent pasting operations without causing blockage and shutdown. These particles not only disrupt the normal operation, but can greatly increase the amount of scrap produced.
Attempts have been made for over 35 years to continuously produce battery paste, preferably on a demand basis, to acquire the implicit benefits of a continuous process without success. Batch mixing problems have resulted in the lack of uniform water content in the paste, necessary for desired density, viscosity and shear of the paste to permit application of the paste to battery plates in a consistent and reproducible manner. Inadequate mixing action has resulted in delayed and incomplete chemical reaction within mixers, with completion of reaction in the product paste outside the mixers, unless feed rates are extremely low.
U.S. Pat. No. 3,576,675 granted Apr. 27, 1971 to Ford Motor Company discloses a method and apparatus for continuous mixing and production of battery paste in which the feed rate of dry particulate lead oxide fed to a mixer housing is monitored and quantities of water and sulphuric acid are sequentially injected into the mixer housing as functions of the weight of lead oxide. The mixing apparatus includes a pair of parallel rotating shafts having mixing paddles mounted thereon separated by alternating stationary flow control discs to direct paste constituents radially across the mixer housing in a slow rolling action as they move through the housing. Notwithstanding the long-felt need for continuous mixing and production of battery paste for consistency of composition and viscosity, the technology of U.S. Pat. No. 3,576,675 has not been commercially used.
Fibres such as polymer or modacrylic fibres are typically added to the battery paste in an amount of about 0.03% to 0.15% of the lead oxide to improve the mechanical strength of the plate and to reduce cracking of the active mass. Some fibres are difficult to disperse, resulting in poor plate strength as well as problems in the pasting machine due to fibre clumping. Further, negative effects include pulling out and distortion of pellets from the plates during subsequent handling. These problems are exaggerated as greater amounts of fibre are added. Long fibres, desirable for better strength, are even more difficult to mix in batch systems and, therefore, are avoided by battery manufacturers. Polypropylene fibres are particularly difficult to mix into paste in batch systems and are therefore seldom used.
Particulate carbon in the form of organic carbon powder, activated carbon powder, or graphite powder, flakes or spheres can be added with other constituents, such as carbon-based expanders, to improve conductivity of negative battery plates in all lead-acid battery types (SLI, industrial, etc.). Presently, carbon is added at a level of 0.3% to 1% by weight of the lead oxide. In some cases, a higher level of carbon would be desirable, but this is not practically possible in the conventional batch system because the carbon cannot be incorporated and dispersed properly for a homogeneous mix, due to the wide difference in density between lead oxide and carbon and due to clumping and caking of the carbon, resulting in unacceptable pastes.
There are many different types of lead-acid batteries and many more different applications. Due to the diversity of the product, some battery manufacturers use many different types of additives. One such additive is tetrabasic lead sulphate seed crystals, again available in several different forms. These additives shorten the curing process and assure proper curing and conversion of the active material to a desired tetrabasic lead sulphate crystal make-up with proper crystal size, shape and distribution. This controlled crystal morphology adds strength to the active mass of the battery plate and increases the life of the product. The additives help to seed the growth of the tetrabasic lead sulphate crystals, providing seed points for the growth of these crystals. In normal batch mixers, the distribution is less than desired, leaving large areas deprived of seed, while other areas are enriched with seed resulting in growth reactions being too great and reaction chemicals being depleted before the reaction can complete the formation of the crystals as desired. This causes variations in the percentage of overall tetrabasic lead sulphate crystals and affects the size and structure of the crystals.
The presence of small amounts of tetrabasic lead sulphate in battery paste often is desired as a seed in the curing stage of the battery-production process. Tetrabasic lead sulphate cannot be readily produced or controlled in batch processes and accordingly is added to the paste in conventional batch processes.